High-temperature furnaces, functioning as reactors, may be used as a reaction chamber to create fine dimension structures, such as integrated circuits, on a semiconductor substrate. Several substrates, such as silicon wafers, may be placed on a substrate holder, such as a substrate rack or boat inside the reactor. Alternatively, a single substrate may be placed on a substrate holder such as a substrate susceptor inside the reactor. Both the substrate and holder may be heated to a desired temperature. In a typical substrate treatment step, reactant gases are passed over the heated substrate, causing the deposition of a thin layer of the reactant material or reactants of the gases on the substrate.
A series of such treatment steps on a substrate is called a recipe. If the deposited layer has the same crystallographic structure as the underlying silicon substrate, it is called an epitaxial layer. This is also sometimes called a monocrystalline layer because it has only one crystal structure. Through subsequent deposition, doping, lithography, etch and other processes, these layers are made into integrated circuits, producing from tens to thousands or even millions of integrated devices, depending on the substrate size and the circuits' complexity.
Various process parameters are carefully controlled to ensure the high quality of the resulting layers. One such critical parameter is the substrate temperature during each recipe step. During CVD, for example, the deposition gases react within particular temperature windows and deposit on the substrate. Different temperatures also result in different deposition rates. Accordingly, it is important to accurately control the substrate temperature to bring the substrate to the desired temperature before the treatment begins.
One factor which critically affects the throughput of a processing reactor is the substrate temperature ramp rate. Such temperature ramping can be required at several points during a given recipe. For example, a cold substrate must be heated to the appropriate treatment temperature. Also, the recipe may require different temperatures for different treatment steps. At the recipe's end, the substrate ordinarily is cooled to a level that the substrate handling device can tolerate. The heating and cooling steps can represent a significant percentage of the processing time and can limit the reactor's throughput. The time between the steady state temperatures is essentially time which should be minimized so as to increase the reactor's throughput.
Accordingly, there may be a need for controlling substrate temperatures with a high power and controllability.